In the field of well logging, there is a clear need for high-resolution borehole measurements. Log analysis in general, and in particular the calculation of hydrocarbon saturations, are often based on induction measurements with an intrinsic resolution of 2 feet or worse. In an inhomogeneous reservoir with sand and or shale layers, the non-linear (current-seeking) nature of the induction measurement becomes evident. If the induced currents run parallel to the bed boundaries (measuring horizontal resistivity, Rh), then the electric currents tend to concentrate in the conductive shale layers, resulting in pessimistic estimates of hydrocarbon saturations when using traditional log analysis. Conversely, currents crossing bed boundaries encounter higher resistivities (vertical resistivity, Rv), sometimes resulting in overly optimistic estimates for hydrocarbon saturation. It is not trivial to reconcile Rh and Rv because shales exhibit different conductivities parallel to and normal to their compression direction. This extra free parameter would then need to be obtained from additional measurements, such as core analysis, which is often not available.
In general, well logging tool responses on a scale equal to or smaller than 2 feet may be considered high-resolution because such tools may be helpful in the disambiguation of mono-axial or tri-axial induction log responses. Electric and acoustic borehole images achieve this resolution, but such images are only available in a fraction of currently drilled holes, and are currently acquired only over limited sections. Oil-based mud poses significant challenges for electric borehole imaging, and an LWD (Logging While Drilling) oil-based imager is not, at this time, known to exist. The reconciliation of borehole imaging with induction measurements involves the summation of sand fractions to compute the net height of the hydrocarbon column over a given interval. Multiplying the net height by effective porosity, hydrocarbon saturation, and lateral area, gives (at least in theory) the total in-place hydrocarbon volume.
NMR (Nuclear Magnetic Resonance) measurements greatly streamline the calculation of hydrocarbon-in-place. As a linear measurement, NMR responds predictably to sand and or shale mixtures in arbitrary bedding and borehole geometries. For layered systems on scales less than the NMR resolution (2 to 4 ft), effective porosity from NMR represents the product ΦH (porosity Φ times column height H), bypassing the summation of individual layers. As a shallow measurement, NMR operates in the flushed or invaded part of the formation. Under irreducible conditions, the “movable” or “free” porosity determined by NMR in the flushed zone equals the porosity fraction available for hydrocarbon accumulation in the formation.
Classically, NMR porosity and irreducible water volume, BVI (Bulk Volume Irreducible), as determined from NMR, are used to derive a first-order estimate of formation permeability. This transform, the so-called Coates equation or the Timur-Coates equation, is highly nonlinear and is rooted in a distributed-shale model. It performs poorly in layered formations with characteristic scales smaller than the NMR resolution. In such formations, large flow volumes may be sustained by thin beds, resulting in high kH values (permeability k times column height H), while the Coates model predicts poor flow based on the large amounts of shale present. Thus, kH prediction from NMR should benefit immensely from improved vertical resolution.
Looking forward, convergence of the complementary features of tri-axial induction and NMR appears likely. One way to make a highly integrated NMR-plus-tri-axial induction evaluation work is for NMR to match at least the 2-foot induction resolution under arbitrary borehole and logging speed conditions. Preferably, NMR should also probe formation in-homogeneities in the 1-foot and ½-foot resolution range to de-convolve the induction response.
There are fundamental limits to borehole NMR arising from signal strength, thermal background noise, and the relaxation time, T1. These constraints led experimentally to a 2-foot antenna in the MRIL® Prime tool from Halliburton Energy Services, Inc. (MRIL®, Magnetic Resonance Imaging Logging, is an NMR wireline tool, and is a registered trademark of Halliburton Energy Services, Inc.) The signal from the antenna for the MRIL tool is averaged several times (stacked) to arrive at an acceptable signal-to-noise ratio (SNR). Depending on logging speed and the interval between measurements (constrained by T1), the vertical interval over which stacking occurs may reach several feet. Overall, the standard log resolution is about one-half (corresponding to 4 ft) of what would be achievable in a stationary measurement (2 ft). Reducing the antenna length provides no improvement in log resolution because the lower raw SNR requires more stacking The choice of a 2-foot aperture proved to be fortuitous with respect to measurements on hydrocarbons, which require that more-or-less the same measurement volume is available to measurements spaced 1-2 seconds apart. This requirement is met with the MRIL antenna at moderate logging speeds.
Schlumberger has developed a CMR (Combinable Magnetic Resonance) wireline logging tool that may be said to have a high-resolution flavor to it. The antenna for the CMR tool is only 6 inches tall, and requires a stacking depth of six to achieve an acceptable SNR. The instrument has only a single measurement volume, requiring either very long wait (idle) times between consecutive measurements, or a compromise with respect to under-calling porosity in free fluids. Typically, a reduction in CMR porosity is accepted to achieve reasonable logging speeds. The CMR tool is often run with pre-set wait times tuned to an anticipated logging speed such that measurement volumes are stacked toe-to-head, which alleviates the need to wait out full magnetization recovery. In this mode, vertical resolution is six times 6 inches, or 3 ft, which yields a resolution similar to the MRIL Prime tool.
Although the MRIL Prime and CMR tools have radically different designs, they nevertheless have similar vertical resolutions due to basic physical constraints. This is illustrated in FIG. 1, where various relative responses to a hypothetical chirp formation are computed. The chirp is shown by the top trace, labeled 102. Classically, a chirp is a signal having a linearly time-varying instantaneous frequency, so that as time increases, its instantaneous frequency increases. Because the tools are assumed to be moving at constant velocity, the distance traveled by a tool is proportional to time, so that the x-axis in FIG. 1 may be expressed as the distance that the tool has moved since the beginning of the chirp. Note that at the right margin of FIG. 1, the chirp has the highest spatial frequency, corresponding to having a high value for 3 inches of travel, followed by a low value for 3 inches of travel. For clarity, noise effects have been omitted.
The second trace from the top, labeled 104 in FIG. 1, shows the computed MRIL response, and the third trace from the top, labeled 106, is the CMR response. They are similar due to the amount of stacking required. The bottom trace, labeled 108, shows the CMR in BVI-only mode, which enables much faster sampling. In this mode, the CMR may double its vertical resolution.
Schlumberger's next-generation NMR wireline logging tool is referred to as an MR Scanner, and has three antennas in different tool sections: The main antenna is closely modeled after the MRIL tool and its side-looking version, the MRIL-XL tool, another tool from Halliburton Energy Services, Inc. Two auxiliary antennas on the MR Scanner provide a CMR-style measurement with 4-inch antenna apertures. It is claimed that stacking of these signals is not necessary, and that in operation the high-resolution antennas are fired every 5 inches. Therefore, assuming a single phase-alternated pair, these antennas may potentially deliver 1-foot log resolution. This may be true in vertical, smooth boreholes, but in deviated, rugose boreholes, we believe it is more likely that the solid, long tool body of the MR Scanner tool will force the high-resolution antennas off the borehole wall, resulting in a distorted measurement that is influenced by borehole mud.
A more robust measurement would be highly desirable, i.e., with a distance between tool face and sensitive volume of at least 2 inches instead of 1 inch as provided by the MR Scanner. It would be useful for such a tool to integrate with existing porosity tools, and to replace them wherever the use of chemical sources is not feasible. It would be desirable for the primary tool response to match the induction response on a length scale of 2 feet at any logging speed. Furthermore, it would be desirable for such a tool to probe the 12-inch, 6-inch and 3-inch scales for de-convolution of mono-axial and tri-axial induction logs, as well as improved kM estimates. Also, it would be desirable for the measurements to be independent of borehole angle, and robust against moderate borehole rugosity.
It is believed that consistent NMR log responses matched to the induction resolution would enable tightly integrated answer products, and a new understanding of the formation under investigation. NMR responses with better resolution than induction would feed into the real-time modeling of the induction response, by stabilizing the under-determined inverse induction problem. As a stand-alone answer product, we expect improvements in permeability estimates by orders of magnitude where currently the simple distributed-shale model does not match the sand-shale bedding reality. An integrated answer product would use both the high-resolution NMR information and the tri-axial induction data to estimate formation producability relative to borehole orientation and placement.